feat(toc,explorer): add accessibility for toggle (#1327)

* Restore focus highlight on explorer toggle button.

Remove `unset: all` declaration causing `outline`
property to be unset. This allows the default
browser focus highlight to be shown.

* Fix semantics of expandable sections (explorer, toc).

This adds the appropriate aria attributes for the [disclosure pattern](https://www.w3.org/WAI/ARIA/apg/patterns/disclosure/examples/disclosure-image-description/#javascriptandcsssourcecode) and uses `visibility: hidden` to remove the hidden elements from the focus order without disrupting the animations. Further work is needed on the tree view nodes.

* Run prettier for SCSS files.

quartz/components/Explorer.tsx

@@ -91,6 +91,8 @@ export default ((userOpts?: Partial<Options>) => {
           data-collapsed={opts.folderDefaultState}
           data-savestate={opts.useSavedState}
           data-tree={jsonTree}
+          aria-controls="explorer-content"
+          aria-expanded={opts.folderDefaultState === "open"}
         >
           <h2>{opts.title ?? i18n(cfg.locale).components.explorer.title}</h2>
           <svg